1. Field of the Invention
The present invention generally relates to a semiconductor device, and more particularly to a semiconductor device wherein an oxide film in a birds beak form is formed in a gate insulating film.
2. Description of the Background Art
A LOCOS (Local Oxidation of Silicon) method and an STI (Shallow Trench Isolation) method are generally known as methods for isolating semiconductor elements. According to these isolation methods, the surfaces of the semiconductor substrate that become active regions are exposed by means of etching after the deposition of an isolation oxidation film which becomes an element isolation structure on the semiconductor substrate, and gate insulating films and the like are sequentially formed on those exposed portions.
In this case, however, the sidewall portions of the isolation oxide film are also etched; therefore, there is a possibility that the end portions of the active regions may become indented in the isolation region in the configuration. Thereby, the form of the elements formed in the active regions becomes unstable and this becomes the cause of dispersion in the characteristics of the device. In addition, an electrical field concentration is caused in an indented portion in the case where a gate oxide film is formed in the portion, and a problem arises wherein the electrical characteristics of the device deteriorate. Thus, a manufacturing method for a nonvolatile semiconductor memory device is disclosed in Japanese Patent Laying-Open No. 2000-315738, in order to solve such a problem. In addition to that, Japanese Patent Laying-Open Nos. 10-242264 and 2001-332638 are cited as prior art literatures that disclose manufacturing processes for isolation regions.
According to a method disclosed in Japanese Patent Laying-Open No. 2000-315738, an element isolation region is formed after the formation of a tunnel oxide film and a first polycrystal silicon film forming a part of a floating gate. Then, a second polycrystal silicon film is formed on the first polycrystal silicon film in order to provide a sufficient capacitance coupling between the control gate and the floating gate, and this second polycrystal silicon film extends over the element isolation region.
In addition, an oxidation is carried out so as to generate birds beak in the tunnel oxide film for the purpose of rounding the corners of the element regions. At the time of this oxidation process, the first polycrystal silicon film is oxidized, whereby the end portions thereof are rounded. The portions of the first polycrystal silicon having a rounded form are not removed by means of subsequent vertical etching; therefore, short circuiting is caused between adjacent floating gates. An etching process wherein a plasma oxide film formed in the element isolation region is etched by a predetermined amount by means of isotropic etching is added to the manufacturing process in Japanese Patent Laying-Open No. 2000-315738 in order to prevent the generation of the short circuiting.
As described above, the first polycrystal silicon film is oxidized so as to form birds beak in the tunnel oxide film in order to suppress the electrical field concentration generating in the end portions of the element regions. However, the end portions of the tunnel oxide film, on which birds beak is formed, have a thickness greater than the predetermined thickness at the time when the tunnel oxide film is initially formed.
Thus, in the case where the portion on which the tunnel oxide film having a predetermined thickness is formed is not sufficiently secured, this causes the failure of the obtaining of the desired electrical characteristics in the tunnel oxide film. In particular, as the miniaturization of semiconductor elements has progressed in recent years, the width of active regions has gradually been scaled down while the width of tunnel oxide films formed in active regions has also become narrower. Therefore, it is important to carry out an oxidation process by appropriately controlling the dimension of the birds beak formed in the tunnel oxide film.
According to the methods disclosed in the prior art, however, the birds beak is formed in the tunnel oxide film through the appropriate selection of the oxidation conditions such as temperature conditions. In such a case, the dimension of the birds beak to be formed cannot be sufficiently controlled, and there is a possibility that the desired electrical characteristics may not be obtained in the tunnel oxide film or that electrical field concentration may be caused in both end portions of the tunnel oxide film.
In addition, according to the prior art, isotropic etching is carried out in order to prevent the generation of short circuiting due to the portions of the first polycrystal silicon film that have not been removed by means of vertical etching. However, it is not preferable to provide such a process because this complicates the manufacturing process.